Molten metals, particularly molten aluminum and molten steel, are frequently contaminated by entrained small non-metallic inclusions that give rise to a variety of shortcomings or defects in products manufactured from the molten metal. For example, such inclusions may cause the solidified metal to tear during mechanical working operations, or may introduce pin-holes and streaks in foils and surface defects and blisters into sheets, or give rise to increased rates of breakage during the production of metal wire, etc.
A known analyzer that enables quick measurements of metal cleanliness and provides size and concentration information of the inclusions is the so-called Liquid Metal Cleanliness Analyzer (often abbreviated to “LiMCA”). A conventional LiMCA apparatus may comprise a probe having an electrically-insulating wall means, often in the form of a sampling tube, having a small precisely-dimensioned passage in a side wall. The tube is immersed in the molten metal to be tested and a uniform stream of the metal is drawn by vacuum or pressure through the small passage while a substantially constant electric current is passed through the stream between electrodes disposed respectively inside and outside the tube. The particulate inclusions generally have very high electrical resistivity compared to the molten metal and the travel of a particle through the passage is accompanied by a change in resistance for the electric current within the passage, thereby producing an electrical pulse in the voltage across the electrodes. The number of pulses produced while a fixed volume of metal transits the passage provides an indication of the number of particles per unit volume present within the metal. Furthermore, it is possible to analyze the pulse to determine particle size and size distribution. Generally, the voltage is monitored in real time, but the voltage trace may be recorded and analyzed afterwards and kept for future referral. Examples of typical LiMCA devices are described in U.S. Pat. Nos. 4,600,880, 5,130,639, 4,555,662, and 5,039,935.
For LiMCA apparatus to work effectively, the current flowing between the electrodes must be direct current (DC) and must be kept fairly constant for a sufficient period of time, e.g. 30 seconds or so, to allow for a reliable measurement. Also, the current passing between the electrodes must be kept fairly high, and it is desirable to minimize random electrical noise that can undesirably obscure the desired voltage signal. To meet these requirements, it has been usual to provide the apparatus with one or more rechargeable batteries (for example of the Nickel-Cadmium type), to generate the required DC current during the testing phase. The batteries are recharged between the test cycles when the generation of electrical noise is not important, e.g. using a mains generator or battery recharger. While the use of batteries as the current source can be effective, batteries take a significant time to recharge and require additional equipment to ensure that the recharging takes place properly. They also tend to be heavy, bulky and may have a short operational life if constantly subjected to rapid discharge and recharge cycles. Another problem that conventional apparatus may encounter is the generation of considerable heat, representing a loss of efficiency and requiring extra size and weight for cooling devices or heat sinks.
The use of ultra-capacitors, rather than batteries, as power sources for LiMCA devices has been disclosed in U.S. Pat. No. 7,459,896 which issued to Marcotte et al. on Dec. 2, 2008 (“the Marcotte et al. patent”) (the disclosure of which patent is specifically incorporated herein by this reference). As explained in this patent, ultra-capacitors can be employed as power sources as an alternative to rechargeable batteries. However, ultra-capacitors have a lower volume charge density than rechargeable batteries and cannot therefore supply high currents at constant rates for extended periods of time. In the device of the Marcotte et al. patent, the use of an ultra-capacitor can result in the generation of significant heat and require circuitry that is susceptible to inclusion of electrical noise. This has necessitated complex measures for eliminating noise from the test signal, e.g. by providing three or more electrodes to generate a reference signal for comparison purposes. There is therefore a desire for alternative approaches that enable the use of ultra-capacitors as a current source without attendant disadvantages.
Previous LiMCA designs, particularly those incorporating batteries, have generally employed large ballast resistors and transistors operating in a linear (intermediate) region to maintain steady current generating high heat losses and requiring heat management to keep operating temperatures within a safe region.
It has also been known to incorporate into the apparatus some means of reducing noise in the voltage signal so that the wanted pulses can be detected more reliably. For example, Marcotte et al. employ a three-electrode design to generate an additional signal containing only noise, and then subtracting the signals from each other to reduce the background noise signal.
However, while effective, this increases the size and cost of the apparatus and requires additional circuitry.
There is therefore a need for alternative designs and methods of use of LiMCA equipment.